Two coherent point sources A and B are in phase and separated by exactly 5λ. A circular observation path is centred at the midpoint of AB and has radius greater than AB ÷ 2. An observer moves once around the complete circle. How many positions of maximum intensity are encountered?
Physics · Wave Optics · interferece of light and huygen principle
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Question

Two coherent point sources A and B are in phase and separated by exactly 5λ. A circular observation path is centred at the midpoint of AB and has radius greater than AB ÷ 2. An observer moves once around the complete circle. How many positions of maximum intensity are encountered?

Physics · Wave Optics · interferece of light and huygen principle

G1 Physics · Accepted Answer

The signed path difference ranges from −5λ to +5λ. For maxima, Δ = nλ. The nine interior orders n = −4 to +4 each occur at two positions, giving 18 positions. The extreme orders Δ = −5λ and Δ = +5λ occur at one point each on the source axis. Therefore, total maxima = 18+2 = 20. Hence the answer is 20.